1. Field of the Invention
The present invention relates to transducers, to tools utilizing such transducers and to methods of making and using transducers. In another aspect, the present invention relates to transducers for use in both logging while drilling ("LWD") and wireline applications, to tools utilizing such transducers and to methods of making and using such transducers. In even another aspect, the present invention relates to transducers with minimal reverberation, to tools utilizing such transducers and to methods of making and using such transducers.
2. Description of the Related Art
Well bores are lined with steel casing to prevent collapse of the bore while drilling and to prevent fluid communication between productive oil and gas bearing formations and nonproductive formations, such as those bearing water. A cement slurry is pumped downhole through this casing and displaced up into the annular space between the casing and the borehole wall where it hardens. Once the formations are separated by the cementing process, the desired formations are perforated for production.
Cracks or voids appearing in the cement between the casing and the borehole result in fluid communication problems. For example, water migrating into a producing is zone typically results in decreased production of hydrocarbons and contamination of equipment.
Acoustic imaging tools utilizing transducers are used to evaluate the condition of the cement by directing sonic pulses through the casing, through the cement and to the rock formations. In general, these tools consist of a transmitter capable of providing acoustical impulses, and at least one receiver responsive to acoustical energy, mounted on a support for movement through the length of the well bore. In pulse echo systems, the transmitter transducer is also the receiver. In pitch catch systems, the transmitter and receiver are spaced apart by a fixed distance. As the support is moved through the well bore, the energy from acoustic pulses periodically generated by the transmitter propagates to the wall, reflects (or refracts), propagates back to the receiver, and is picked up at the receiver. The amplitudes of the received signals are correlated with the depth in the well bore to provide a log indicating the qualities of the cement bonding to the bore as a function of depth in the well.
In order to evaluate formation, cement, casing and drilling fluids in wells, high resolution ultrasonic transducers are needed to identify features having small spatial size. This spatial resolution requires that the an transducer be responsive to signals separated by short time intervals. Broad frequency bandwidth is required. Because high frequency signals are highly attenuated, a highly damped pulse with a center frequency having a wavelength slightly smaller that the spatial features of interest is required. However, high damping requires that the transducer have very short reverberation time from any acoustic path within the transducer package.
The performance of logging while drilling caliper tools and the wireline circumferential scanning tools, pulse echo cement bond tools and cement imaging tools are degraded by reverberation in the transducers. If the reverberation can be minimized, the minimum inspection distance can be decreased, permitting nearer standoff measurements, improved bond measurements and detection of thinner cement layers behind casing. Improving the focusing of the open hole scanning transducers will give better estimations of formation surface texture.
Present tools contain transducers having significant reverberations that interfere with signal interpretation. Some open-hole scanning transducers have step focusing which gives reverberations in the lens itself. In addition, focused caliper standoff transducers have undesirable reverberations from plastic lenses. The caliper/standoff transducers have radiation patterns which lose echoes for certain eccentered configurations.
Open hole scanning transducers also need a transducer with less reverberation to operate in heavier muds than can be investigated with the transducers now available. The logging while drilling, formation speed of sound, and caliper standoff tools also need an improved transducer to reduce loss of signal for eccentered tools in the borehole, to investigate smaller standoffs and to handle heavier weight muds.
Therefore, there is still a need for transducers that do not suffer from the deficiencies of the prior art.
There is another need in the art for transducers with minimal reverberation.
There is even another need in the art for transducers that permit nearer standoff measurements, improved bond measurements and detection of thinner cement layers behind the casing.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.